In general, fields of application with significant fluctuations of a thermal flow in or one a micromechanical device by electromagnetic and/or thermal factors represent substantial fields of application of the invention. Such devices with significant fluctuation of the thermal flow by electromagnetic factors may for example be micromechanical devices with possible employment in the entire electromagnetical spectral range. This means that the micromechanical devices may operate in the visible, in the ultraviolet, in the near-infrared radiation range, in the terahertz range or in the range of soft X-ray radiation up to the radiation in the radio wave range.
Fields of application of the present invention may be devices for significant electromagnetic power densities. Such devices inherently exerting thermal action on the micromechanical device may for example be devices for the photolithography, the LIDAR earth recognition, or devices for LED, laser or maser fields of application.
Further fields of application of the present invention with significant fluctuations of the thermal flow by thermal factors, for example, represent micromechanical devices with high temperature dynamics, i.e. for example devices for the employment in the automobile technology, the satellite technology, the medial technology, the low-temperature sensor technology, the vacuum technology, or for sensor and actor devices.
Thermal stresses of reflective-micromechanical devices may lead to the alteration of important characteristic device quantities. In reflective-micromechanical devices, micromechanical-optical key parameters, such as the resonance frequency, the deflection position or the mirror planarity in the case of a scanner mirror, may change. The variations of characteristic system quantities, such as the mapping fidelity, the positioning speed, etc., resulting therefrom necessitate adequate control of the thermal flow when employing the micromechanical devices, in particular for laser precision applications. This problem of thermal instability of reflective-micromechanical devices has been known for several years.
Known methods for avoiding thermal instabilities in reflective-micromechanical devices include operating the reflective-micromechanical device at sufficiently low electromagnetic field densities, which for this reason do not cause any significant thermal input into the micromechanical device, for example.
A further possibility of reducing thermal instabilities in micromechanical devices due to the interaction of the micromechanical device with electromagnetic radiation is achieved by Sandner et al. (“Highly reflective optical coatings for high power applications of micro scanning mirrors in the UV-VIS-NIR spectral region”, Proceedings SPIE, Vol. 6114 (2005)) by the generation of a high surface reflection with anti-reflection coating layers for the reduction of the thermal coupling.
Alternatively, thermal instabilities may also be achieved by a special illumination scheme in the laser operation, as this is described in WO 2005/015903 A1.
As described in the patent specification WO 2005/078506 A1, a thermal compensation flow can be generated with a localized electrical heating in different areas of the device for the adjustment of a stabilized temperature in a micromechanical device.
A temporally limited operation of the micromechanical device with “rest times” may also be performed to avoid or minimize significant thermal heating.
A further approach for diminishing thermal instabilities in reflective-micromechanical scanners is based on the special design of mechanical springs for the suspension of the scanner with optimized heat removal, or on a gas purge of the micromechanical device for heat removal.
Basically, the previously known approaches for thermal stabilization of micromechanical devices have different difficulties and/or disadvantages.
For example, the above-mentioned design changes of the mirror springs for scanner mirrors may not lead to optimum mechanical properties of these thermally optimized mirror springs.
A local electrical heating for temperature stabilization of a micromechanical device may react to temperature changes only partially and maybe with a significant temporal offset. Moreover, major thermal flow variations can only be regulated in limited cases. The operation of the micromechanical device with “rest times” places significant limitations on the operating speed of the micromechanical device and therefore is not desirable.
A special laser illumination scheme for avoiding thermal instabilities in reflective-micromechanical devices, as mentioned above, may partially be employed for the control of the thermal flow in the device. Such an illumination scheme potentially is limited, however, in the temporal and spatial dynamics with respect to the thermal flow to be compensated.
Power demands of industrial users for high precision, for example of the resonance frequency of micro-scanner mirrors in laser applications or stable operation under climatically varying conditions of application, still remain limited, however, with the above-mentioned measures for temperature stabilization.
A spatial and temporal exact stabilization of the thermal flow within the micromechanical device can be achieved only partially at present. Substantial limitations are determined by micromechanical motion elements acting almost thermally insulating, such as the mirror springs in scanner mirrors.